As far as autostereoscopic display devices are concerned, switching between autostereoscopic and two-dimensional representation is known for example from WO 2005/027534, “Autostereoscopic multi-user display”, filed by the applicant. It includes a focusing element for focusing a preferably white, homogeneous light distribution on to the eyes of observers, and a transmissive image matrix, which can be freely controlled as regards colour and intensity, and which is permeated by the light of the focusing element, for sequential representation of images or image sequences with monoscopic or stereoscopic image contents. The focusing element is a sweet spot unit which is disposed in front of the image matrix, seen in the direction of light propagation, where the light emitted by said focusing element permeates a preferably large region of the image matrix and can be focused so to form a sweet spot with an individually defined extent on positions which correspond with the observer eyes. Temporal multiplexing makes it possible to switch the entire display or just selectable regions of it from autostereoscopic to two-dimensional representation. Moreover, multiple observers can be provided different image contents.
WO 2005/060270 filed by the applicant, “Autostereoscopic multi-user display”, describes a display device which is characterised by spatial multiplexing. The display device includes a sweet spot unit which is direction-controlled by a tracking and image controller, said sweet spot unit consisting of an illumination matrix with a plurality of illumination elements which can be discretely activated, and a projection device for projecting alternately active illumination elements in the form of directed bundles of rays on to extended sweet spots which correspond with different eye positions, so that right and left images of a stereoscopic image sequence provided on a transmissive image matrix can be rendered visible at right/left observer eye positions. In order to be able to switch from autostereoscopic to two-dimensional representation, the spatial multiplexing is compensated by interleaving respective image contents on the image matrix. Again, multiple observers can be provided different image contents.
WO 2005/011291, filed by Philips, describes a switchable autostereoscopic display. It contains a switchable lenticular which is filled with an LC liquid and which is surrounded by a different material with a refractive index of greater than one. The refractive index of the LC liquid is controlled with the help of an electric field and can be identical to or different from the refractive index of the surrounding material. If the refractive indices differ, the light is diffracted at the interface; if the refractive indices are the same, the light passes the interface without diffraction. The lenticular can be thus be turned on and off. In the on state, an autostereoscopic representation is provided due to the active image separation means, otherwise a two-dimensional representation is provided.
WO 2004/070451, filed by Ocuity, describes a switchable autostereoscopic display with a switchable lenticular. The lenticular consists of a birefringent material and is surrounded by an isotropic material with a refractive index that is identical to that of the polarisation direction of the lenticular. The light of one polarisation direction thus passes the lenticular without being diffracted, while the light in the perpendicular polarisation direction is subject to a lens effect. Behind the lenticular, there are disposed optical components which only transmit the light of the one or of the other polarisation direction. It can thus be selected whether the observer sees the light with or without lens effect, and thus whether he watches a two-dimensional or an autostereoscopic representation.
A device for reconstructing video holograms, in short a holographic display, typically contains an SLM with an arrangement of controllable pixels which reconstruct object points by electronically influencing the amplitude and/or phase of illuminating light. Such an arrangement is a form of a spatial light modulator SLM. An SLM may for example also be a continuous SLM instead of a matrix SLM, including a continuous SLM with matrix control or an acousto-optic modulator AOM. A liquid crystal display LCD is an example of such a suitable display device for the reconstruction of video holograms by way of spatial amplitude modulation of a light pattern. However, the principle can also be applied to other controllable devices which take advantage of coherent light to modulate a light wave front. A pixel is individually addressed and controlled by a discrete value of a hologram point. Each pixel represents a hologram point of the video hologram. In an LCD, the term ‘pixel’ is therefore used for the individually addressable image points of the display screen. In a DLP, the term ‘pixel’ is used for an individual micro-mirror or a small group of micro-mirrors. In a continuous SLM, a ‘pixel’ is the transitional region on the SLM which represents a complex hologram point. The term ‘pixel’ thus generally denotes the smallest unit which is able to represent or to display a complex hologram point.
As far as holographic displays are concerned, in WO 2004/044659 the applicant describes a device for the reconstruction of video holograms. It comprises an optical system that consists of at least one real or virtual point or line light source which emits sufficiently coherent light and a lens, as well as the video hologram, which is composed of cells arranged in a matrix or in an otherwise regular pattern with at least one opening per cell, the phase and/or amplitude of said opening being controllable, and an observer plane which coincides with the image plane of the light source, where an observer window is disposed in the observer plane in a periodicity interval of the reconstruction in the form of a Fourier transform of the video hologram, while the reconstruction of a three-dimensional scene can be watched through that observer window, the extent of said observer window not being greater than the periodicity interval.
According to WO 2006/027228 filed by the applicant, “Method and device for encoding and reconstructing computer-generated video holograms”, the display includes a line light source which emits light which is sufficiently coherent in one direction, and focusing optical means, in order to holographically reconstruct a scene in frustum-shaped reconstruction spaces with observer windows, after modulating the light emitted by the light source by controllable pixels which are arranged in a matrix. It is characterised in that the line light source is disposed horizontally, so that its light exhibits sufficient coherence in the vertical direction, and in that the controllable pixels are coded in pixel columns such that there is one column group for each eye of an observer as one-dimensional, vertically encoded holograms of the same scene, where the two column groups are interleaved horizontally, and where there are image separation means with separating elements which are disposed parallel to the pixel columns, said separating means releasing one column group for the respective observer eye and blocking it for the other observer eye.
Both holographic displays are based on the idea not to reconstruct the object of the scene, which can then be watched by the observer, but to project into two small observer windows, which cover the pupils of the observer eyes, the wave front which would be emitted by the object of the scene if it existed in reality at the given location. The former holographic display device is characterised by temporal multiplexing, the latter by spatial multiplexing.
An ‘observer window’ is a limited virtual region through which the observer can watch the entire reconstruction of the three-dimensional scene at sufficient visibility. The observer window is situated on or near the observer eyes. The observer window can be displaced in the x, y and z directions. Within the observer window, the wave fields overlap such that the reconstructed object becomes visible for the observer. According to an embodiment of this principle, the scene can be observed through the observer window and is reconstructed in a frustum which stretches between the edges of the observer window and the SLM. It is possible to use two observer windows, one for each eye. Generally, more complex arrangements of observer windows are possible as well. It is further possible to encode video holograms which contain objects or entire scenes which appear behind the SLM for the observer. The virtual observer windows can be tracked to the actual observer position with the help of known position detection and tracking systems.
In this document, a light source is considered sufficiently coherent if the light is spatially coherent to a degree that it allows interference, so that it is at least suitable for a one-dimensional holographic reconstruction with an adequate resolution. Spatial coherence concerns the lateral extent of the light source. Light sources such as LEDs or fluorescent lamps can fulfil this requirement if their light falls through a sufficiently narrow opening. Light of a laser light source can be considered as emitted by a point source within diffraction limits. It will result in a sharp reconstruction of the object, i.e. each object point is reconstructed as a point within diffraction limits. Light of a spatially incoherent light source has a lateral extent, thus leading to a diffuse and blurred reconstruction of the object. The degree of diffusion or blur is defined by the lateral extent of an object point which is reconstructed at a certain position. In order to be able to use a spatially incoherent source for the reconstruction of a hologram, a compromise must be made between the reconstruction quality and the brightness by adjusting the width of the opening accordingly. A narrower opening improves the spatial coherence, and thus reduces the degree of diffusion and blur. However, a narrower opening also reduces the brightness. The term ‘partial spatial coherence’ is used to describe such a light source. Temporal coherence concerns the spectral bandwidth of the light source. In order to ensure temporal coherence, the light must have an adequately narrow wavelength range. The spectral bandwidth of highly bright LEDs is sufficiently small so to ensure temporal coherence for holographic reconstructions. The diffraction angle at the SLM is proportional to the wavelength, so that only a monochromatic light source permits a sharp reconstruction of an object point. A broad spectrum leads to widened object points and diffused and blurred object reconstructions. The spectrum of a laser source can be considered monochromatic. The spectral bandwidth of a. LED is sufficiently small to produce good reconstructions.
In the above-mentioned holographic displays, the coded hologram forms the transform of the three-dimensional scene which is to be reconstructed. The term ‘transformation’ shall be construed such to include any mathematical or computational technique and any approximation method which is identical to a transformation. Transformations in a mathematical sense are merely approximations of physical processes, which are described more precisely by the Maxwellian wave equations. Transformations such as Fresnel transformations or the special group of transformations which are known as Fourier transformations, are second-order transformations. As they are substantially algebraic and not differential, they can be handled efficiently using common computing means. Moreover, they can be implemented precisely by optical systems.
As holographic displays are in an early stage of development and prototyping, no holographic display devices are hitherto known which can be switched to autostereoscopic or two-dimensional representation modes.
It is the object of the invention to provide a method for holographic displays to ensure multimode representation and to provide holographic, autostereoscopic and two-dimensional representation modes. The individual representation modes shall be available alternatively or in any interleaved pattern and for multiple observers. Further, a holographic display shall be provided which can be used to implement the novel method.